The present invention relates generally to interface devices between humans and computers, and more particularly to computer interface devices that provide force feedback to the user.
Computer systems are used extensively in many different industries to implement computer controlled simulations, games, and other application programs. More particularly, these types of games and simulations are very popular with the mass market of home consumers. A computer system typically displays a visual environment to a user on a display screen or other visual output device. Users can interact with the displayed environment to play a game, experience a simulation or "virtual reality" environment, or otherwise influence events or images depicted on the screen or in an application program or operating system. Such user interaction can be implemented through a human-computer interface device, such as a joystick, "joypad" button controller, mouse, trackball, stylus and tablet, foot or hand pedals, or the like, that is connected to the computer system controlling the displayed environment. The computer updates the game or simulation in response to the user's manipulation of a moved object such as a joystick handle or mouse, and provides feedback to the user utilizing the display screen and, typically, audio speakers.
Force feedback interface systems, also known as haptic systems, additionally provide force feedback to a user of the system. In a typical configuration, a host computer implements software such as an application program, simulation, or game and communicates with a connected force feedback interface device. The user grasps a user object of the interface device, such as a joystick, mouse, steering wheel, stylus, etc., and moves the object in provided degrees of freedom. The movement of the user manipulatable object is sensed by the host computer using sensors, and force sensations controlled by the host computer are provided to the user object using actuators of the force feedback interface device. Force feedback can be effectively used to simulate a variety of experiences, from a crash in a vehicle, a gun firing, a bumpy road, etc., and can thus supply the mass market of computer users an entirely new dimension in human-computer interaction.
Force sensations are created for the user of force feedback interface systems often by using actuators such as active motors that generate a torque on a rotating shaft. Nearly all common types of motors create torque through the interaction of a static magnetic field created by permanent magnets and a variable magnetic field created by electric current flowing through metallic (e.g., copper) windings. These magnetic fields are commonly directed through the stationary part of the motor (stator) and the rotating part of the motor (rotor) through ferrous structures. Brush-commutated (or "brush-type") DC motors, stepper motors, and brushless DC motors are common examples of this type of permanent magnet/iron arrangement.
One problem presented by prior art force feedback and haptic systems is the "cogging" effect occurring with the use of these types of motors. Cogging is the term used to describe the tendency of a motor rotor to align itself preferentially with the stator. In a typical brush-type DC motor, there may be multiple positions per shaft revolution where the motor rotor prefers to rest. This effect is sometimes described as "detenting" or "ratcheting" and can result in substantial variation in the output torque of the motor, both when powered and unpowered. Cogging is fundamentally caused by the change in reluctance of the magnetic flux path: the preferential positions are essentially "reluctance minimization points" where the energy stored in the magnetic circuit is at a minimum.
Cogging can be particularly problematic in force feedback devices because the motor rotational speed is so slow that the user is often able to feel each individual torque disturbance as the user manipulatable object is moved and/or as forces are output on the user object. This effect is often perceived by users as "roughness" or "pulsations" and is usually equated with poorly-performing force feedback or haptic systems. The cogging effect problem becomes even more acute in force feedback systems in which forces must be amplified by a mechanical transmission system, such as a capstan drive system, to provide users with realistic force environments, since the cogging effect is amplified as well as the forces. If low friction, high stiffness transmission systems are used, the cogging effect also is quite noticeable to the user, unlike in high friction systems such as gear systems which produce enough noise themselves to mask the cogging effect. Thus, the cogging effect reduces the realism and fidelity of forces in high bandwidth force feedback systems which are used to accurately transmit forces to the user object.
Some prior art solutions to the cogging effect have been devised. In some applications, brushless types of DC motors are used, in which the cogging effect is not as severe as in brush-type DC motors. However, brushless motors are far more expensive and complex than brush-type motors and require more sophisticated control circuitry, and thus are not as viable an option in the low-cost, mass-consumer force feedback market. Cogging can be minimized in brush-type motors by altering the magnetic design of the motor. For example, in a brush-type DC motor, a common way to minimize cogging is to simply increase the number of slots or teeth on the rotor. This approach usually reduces the amplitude of the cogging while increasing the frequency of the "pulsations." This method is expensive since more coils and teeth must be provided. Reduction of the cogging effect can also be accomplished by carefully designing the shape and size of the tips of the rotor teeth and the magnets. Through detailed analysis and design, this approach can yield some improvement, through it is usually rather small. However, neither of these techniques reduces the cogging effect to desirable levels to allow satisfactory use of low-cost motors in high bandwidth force feedback interface devices.
Another important concern for a force feedback interface device is communication bandwidth between the controlling computer and the interface device. To provide realistic force feedback, the complex devices of the prior art typically use high speed communication electronics that allow the controlling computer to quickly send and update force feedback signals to the interface device. The more quickly the controlling computer can send and receive signals to and from the interface device, the more accurately and realistically the desired forces can be applied on the interface object. However, a problem is evident when prior art force feedback interface devices are provided to the mass consumer market. The standard serial communication interfaces on personal computers and video game consoles are typically quite slow (i.e., have low bandwidth) compared to other communication interfaces. Realistic and accurate force feedback thus becomes difficult to provide by a controlling computer system to a prior art interface device connected through such a serial interface.
Finally, mass market force feedback interface devices must necessarily be low cost devices that are simple to manufacture so that the device is competitively priced in the high volume, aggressive home computer and home video game markets.